1. Field of the Invention
The present invention relates to a wavelength division multiplexed (WDM) transmission path which links a dispersion-shifted fiber with a dispersion-compensating fiber.
2. Description of the Related Art
The lowest transmission loss in a quartz-type optical fiber is at wavelengths near to 1550 nm, and this wavelength band is conventionally used in long distance transmissions. Generally, a dispersion-shifted fiber with chromatic dispersion having a low absolute value in the 1550 nm band is used as the transmission path in this case. With the recent demand for large-capacity optical communications, there has appeared an optical communication system which uses high-power signal light, obtained by WDM signals and using an optical amplifier.
On the other hand, when the chromatic dispersion value of the optical fiber in the used waveband reaches zero, four-wave mixing, which is one type of nonlinear optical effect, is liable to occur; this is not desirable in WDM transmission. For this reason, a dispersion-shifted fiber having a zero-dispersion wavelength near 1550 nm is not suitable for use in WDM transmission in a waveband centered on 1550 nm.
Accordingly, an NZDSF (non-zero dispersion-shifted fiber) having zero-dispersion wavelength near 1520 nm, or near 1580 nm, has been developed.
Furthermore, a standard single mode optical fiber having zero-dispersion wavelength near 1300 nm has been widely used conventionally, and there have been proposals to a WDM transmission path which links this standard single mode optical fiber with the dispersion-compensating fiber. For example, Japanese Unexamined Patent Application, First Publication No. Hei 10-319266 discloses a hybrid optical fiber link in which, by linking the standard single mode optical fiber with the dispersion-compensating fiber, in a waveband of 1500 nm to 1600 nm, positive chromatic dispersion (accumulated dispersion) of the standard single mode optical fiber is cancelled by the negative chromatic dispersion of the dispersion-compensating fiber, whereby the dispersion of the entire transmission path is almost zero.
Furthermore, given the demand for even larger capacity optical communication, there have been announcements of investigations of not only the waveband of near 1530 nm to 1570 nm used in conventional optical communication systems, but also wavebands such as 1570 nm to 1630 nm and 1460 nm to 1530 nm. These wavebands, which are currently in use and under consideration, are generally termed S-band (1460 nm to 1530 nm), C-band (1530 nm to 1570 nm), and L-band (1570 nm to 1630 nm).
However, since the conventional NZDSF has a zero dispersion wavelength near to 1520 nm or 1580 nm, noise is liable to be generated by four-wave mixing in these wavebands. Therefore, the NZDSF has a drawback of not being suitable for WDM transmission in the S-band and L-band.
Although the zero dispersion wavelength of the standard single mode optical fiber is not within any of the S-band, C-band, and L-band, the standard single mode optical fiber has high chromatic dispersion in these wavebands, and, due to the effect of accumulated dispersion (i.e. residual dispersion) when transmitting optical signals over long distances, has a drawback of waveform deterioration caused by interference between adjacent signals. There is a further drawback of restrictions on the transmission distance in the L-band, where residual chromatic dispersion is high.
For the reasons given above, there has still not been disclosed a WDM transmission path which can select any waveband from the wide wavebands represented by S-band, C-band, and L-band.